During an interview on HBO's "Real Time with Bill Maher," Steyer formally challenged the conservative Koch brothers — who made their fortunes in the fossil-fuel industry — to a debate on energy policy, specifically Keystone, subsidies for the oil industry, and climate change.

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"Today I am issuing a formal invitation to Charles and David Koch to come out of the shadows and join me in exactly what they’ve requested: a free and open debate," Steyer said Friday night.

He added that the Kochs have long "bemoaned" the oppression of open debate, and chided their ability to drown out others with the use of "expensive megaphones."

Still, many have compared Steyer's rise to the forefront of U.S. politics to that of the Kochs, as the billionaire climate activist has vowed to use his pocketbook to make climate change a wedge issue in coming elections.

But Steyer is pushing back against that parallel.

“Make no mistake: there is a significant difference between what NextGen Climate is doing and what the other side—led by special interests like the Koch Brothers—is doing," Steyer said. "We are using our resources to promote an interest that we believe will help our children, while they appear to be promoting an agenda that will benefit their economic self-interest."

Steyer founded his political action committee, NextGen Climate, in 2013. Since, he has attracted much attention from both parties over his disdain for the Keystone XL pipeline, which would carry crude from Alberta oil sands to Gulf refineries, and his willingness to help Democrats maintain control of the Senate.